In ion implantation and other processes in which a beam of particles or radiation is directed at a workpiece, the beam must generally be moved across the workpiece in a controlled manner to spread the particle or radiation dosage across the workpiece. In doping semiconductor wafers, a common technique is to move the wafers relative to a fixed beam along two orthogonal directions. The wafers are supported on a moving surface, which moves them at high speed along a scanning direction and at a slower speed along an orthogonal control direction. To achieve uniform doping density, it is conventional to measure the beam intensity, and vary the control speed accordingly, increasing the control speed when the intensity of the beam on the wafers increases and decreasing the speed when it decreases (see e.g. my patent, U.K. Pat. No. 1,389,294).
In the prior art (e.g., Robertson U.S. Pat. No. 3,778,626), beam intensity has been detected by measuring the current flowing to or from the wafer support element. The support element is electrically isolated from the rest of the apparatus, and a current-measuring lead is connected to the element via a slip ring or other sliding electrical contact, all of which complicates the design. Furthermore, this measurement is sensitive to errors from back scatter of secondary charged particles from the surface of the wafers and support element. Ko et al. U.S. Pat. No. 4,011,449 discloses placing a Faraday cage ahead of the support element to capture these scattered charged particles. Current is measured at both the Faraday cage and at the support element. Such prior-art devices suffer from the large capacitance created by measuring current at the support element. The capacitance tends to magnify the noise in the current measurement and generally increases the difficulty of making a precise measurement when small currents (e.g., 10 microamperes) are involved. The devices also cannot easily be used in applications wherein a flood of electrons is directed at the workpieces to neutralize the charge of implanted ions. The electron source must be carefully isolated and contained within the Faraday cage ahead of the support element in order that they not alter the current reading generated by the ion beam. Furthermore, these prior art devices are more difficult to shield from stray electric and magnetic fields.
In another technique Shifrin U.S. Pat. No. 4,021,675 discloses another technique in which a Faraday cage is placed ahead of the wafers and support element. The beam is scanned across an aperture in a plate positioned ahead of the support elements, and beam intensity is measured by Faraday cages located at the edges of the aperture. This has the major disadvantage of requiring scanning of the beam.
When the support element is a spinning disk and the orthogonal control direction is the radial direction along the disk, the prior art (e.g., Robertson) has also shown varying the radial translation speed of the disk in inverse proportion to a measurement of the radial position, to thereby correct for the difference in disk area swept per revolution by the beam as the beam moves radially inward on the disk.